🏗 parser: Add scoped typed analysis to declarations (#15)

Add scoped typed analysis to declarations, ensuring that variables or
parameters do not get redeclared in same block. Variables may be
re-declared in inner scope of a new block with a new type which does not
affect the outer scope. Add detailed tests for various scenarios.

As the parser will also take care of type analysis this is a necessary
addition and patches a whole in the previous global type analysis for
redeclaration.

Add a minor test refactor for parse error assertions

This merges the following commits:
* parser: Factor out parseError function in tests
* parser: Add scoped typed analysis to declarations

     pkg/parser/parser.go         | 196 ++++++++++++++++++++---------------
     pkg/parser/parser_literal.go |  18 ++--
     pkg/parser/parser_test.go    | 135 ++++++++++++++++++++++--
     pkg/parser/scope.go          |  34 ++++++
     4 files changed, 282 insertions(+), 101 deletions(-)

Pull-Request: #15